1. Field of the Invention
The present invention relates generally to semiconductor manufacturing processes, and more particularly, to methods and systems for cleaning semiconductor wafers.
2. Description of the Related Art
The fabrication of semiconductor devices involves numerous processing operations. These operations include, for example, impurity implants, gate oxide generation, inter-metal oxide depositions, metallization depositions, photolithography patterning, etching operations, chemical mechanical polishing (CMP), etc. As these operations generate particles and residue, there is a need to clean wafer surfaces, thus removing the contaminants such as adhered particles and adsorbed compounds (e.g., organic and metallic) from the surfaces of the wafer. Contaminants should be removed from wafer surfaces, as the existence of such contaminants has a detrimental effect on the performance of the integrated circuit devices.
Cleaning the front side (i.e., the active side or surface) of the wafer has traditionally been given higher priority in typical wafer cleaning systems and processes. This is because contaminants on the active side of the wafer can directly cause deleterious defects in the processing of the wafer. As wafer sizes have increased and/or feature sizes have decreased, certain shortcomings have been associated with lack of adequate and proper processing of the back side (i.e., non-active side) of wafers. One such limitation is the deviation of focal plane during the processing of wafers, which is specifically more pronounced during the photolithography-processing step.
By way of example, when a wafer is held on a vacuum or an electrostatic chuck, the presence of particle contaminants adhered to the wafer back side can cause the formation of high and low points throughout the wafer surface. As a result, the wafer surface is (locally) tilted or distorted, thus creating a focal plane deviation. This deviation, although very slight, can present a number of challenges in the photolithographic processes that form very small features.
In addition to creating focal plane deviation, the contaminant particles have proven to migrate from the back side to the wafer front side. For example, the migration may occur during a wet processing step and/or as the wafer is moved or otherwise handled between the processing or metrology tools. Further, the back side contaminants can also migrate from one process tool/step to contaminate subsequent processes.
In an attempt to eliminate such drawbacks, double-sided cleaning processing tools have been implemented. One of such double-sided tools is a brush scrubbing tool, which includes a pair of symmetrical brushes. FIGS. 1A and 1B illustrate two types of prior art, two sided wafer scrubbers. FIG. 1A shows a horizontally orientated, conventional wafer scrubber 100. FIG. 1B shows a vertically oriented, conventional wafer scrubber 100′. Both FIGS. 1A and 1B include a pair of brush cores 102a and 102b each having been mounted by a corresponding brush 104a and 104b. As shown, the outer surface of each of the brushes 104a and 104b is covered with numerous nodules 105a and 105b, respectively. However brushes 104a, 104b can also have a substantially smooth surface (i.e., without significant nodules 105a, 105b). In both orientations, a wafer 106 is scrubbed as the symmetrical brushes 104a and 104b rotate in a corresponding rotation direction of 108a and 108b. 
As shown, the physical makeup of the brush cores 102a and 102b are identical. Similarly, the outer surfaces of the brushes 104a and 104b are constructed from the same material, thus having the same density and compressibility as well as surface contact areas with the wafer. Therefore, in both configurations of prior art wafer scrubbers, an identical amount of pressure is applied to each of the contact areas as the brushes 104a and 104b are respectively applied to the front and the back side of the wafer 106 with identical forces. Consequently, in prior art wafer scrubbers, the front side and the back side of the wafers are treated identically, even though only the front side of the wafer includes active components.
FIG. 1C shows a cross-section of a typical semiconductor wafer 150. The wafer has multiple layers 154–160 of material built up on a wafer substrate 152. A via 164 is shown etched through the layers 160, 158, 156 to expose a copper layer 154. Typical back side contaminants 170 are particles and byproducts from a previous process step are shown trapped between the wafer substrate 152 and an electrostatic chuck 180.
The conventional wafer cleaning systems require separate compartments or cleaning process tools for scrubbing and rinsing or a pre-scrubbing treatment. The separate cleaning tools are required because often a desired rinsing or pre-scrubbing solution may not be compatible with the material of the scrubbing rollers such as the scrub brushes 104a, 104b. The incompatible rinsing or pre-scrubbing solution may be desired to soften, loosen or dilute the contaminates to be removed from the wafer. A rinsing solution may also be applied to the wafer after the wafer has been scrubbed. For example, an aggressive pre-scrubbing solution may deteriorate the scrub brushes 104a, 104b. Therefore, multiple cleaning and rinsing processes and corresponding process tools are required to separate the use of the desired rinsing or pre-scrubbing solution from the scrub brushes 104a, 104b so as to not damage the scrub brushes 104a, 104b. The multiple cleaning process tools also require handling the wafer from one process tool to the next, which adds both complexity to the overall semiconductor manufacturing processes and also provides more opportunities for additional contaminants to be deposited on the wafer.
For example, after an etch process, the back side contaminants 170 can include ash and various polymer, particulate and other contaminants. The ash and various chemical contaminants can end up on the chuck 180, which then transfers them to the back side of the wafer 150. In another example, in a physical vapor deposition (PVD) process tool, copper or other metallic layers are deposited on all exposed surfaces inside the PVD tool. Therefore, the particles and chunks of the deposited material can become contaminants 170 on the surface of an electrostatic chuck within the PVD tool. When the next wafer is mounted on the electrostatic chuck 180, the particles and chunks can be transferred onto the backside of the wafer 150. Further, as the wafer is processed in the PVD tool, the particles become firmly attached to and even embedded in the backside of the wafer 150.
The attached particle contaminants and the chemical contaminants can require very aggressive cleaning solutions to effectively remove them from the back side of the wafer. However, the aggressive cleaning solutions may not be acceptable for use on the front side of the wafer. By way of example, if the contaminants 170 on the back side of the wafer 150 include copper or other metallic and polymer particles, contaminants and residue, then an aggressive, copper-reactive cleaning solution might be ideal for rapid and complete removal of the copper particles and residue in the contaminants 170. However, the front side of the wafer 150 may also have exposed copper, such as a copper line or exposed copper 154 at the bottom of via 164. The exposed copper 154 on the front side of the wafer must not be removed or damaged. Therefore, the ideal aggressive, copper-reactive cleaning solution cannot be used and a lesser aggressive cleaning solution must be used instead. As compared to the ideal aggressive, copper-reactive cleaning solution, the lesser aggressive cleaning solution may not fully remove the desired contaminant (i.e., copper), or may require additional cleaning steps, or may require extensive cleaning time to achieve the desired result. Further, while the back side of the wafer may require cleaning with an aggressive cleaning solution, the front side may also require cleaning but with a less aggressive solution. Conventional two side wafer scrubbers 100, 100′ require the same cleaning solution be applied to both sides of the wafer.
In view of the foregoing, there is a need for a system and method of applying cleaning solutions to a wafer, inside a single wafer cleaning process that may not be compatible with the scrubbing rollers.